This invention relates to a method and system for reducing power consumption in a line driver used in telecommunication systems.
Efforts have been made by manufacturers of Digital Subscriber Line (DSL) hardware to reduce overall power dissipation of DSL line cards. Although overall power reduction improvements have been made, significant power improvements in the field of line drivers have evidently not yet been made. Known relatively minor improvements have been due to crest factor reductions that in turn have precipitated a slight driver voltage rail reduction at the expense of processing speed, or so-called DSP MIPS. For all these applications the driver power has not significantly improved.
To make DSL technology attractive, the overall power dissipation must be reduced beyond the presently offered solutions. This power dissipation manifests itself in the form of increased operation temperature of the equipment. A number of designs constraints are introduced in order to maintain circuits at a reasonable operating temperature, including the inclusion of additional fans, air conditioning, heat sinks and space for thermal ventilation. These constraints significantly increase the material, labor and maintenance cost associated with the system. Furthermore, excessive heat may restrict the density of equipment, thereby increasing the size of the facility hosting the system and/or limiting the number of customers that can be served by a fixed size facility. Thus, reducing the power consumption in communications systems can be a key aspect of any system design.
The line driver is a component that consumes a significant amount of power. Typically, the line driver includes an amplifier for receiving an analog signal from a preceding circuit, such as a digital to analog (DA) converter, to drive this signal through a source resistance and a line transformer onto a twisted pair telephone line or loop. The DSL line drivers commonly in use include a source resistance equal to the reference impedance of the loop, usually 100 ohms implemented as a series resistance. Typical line drivers use two amplifiers working differentially. A factor that exacerbates driver design is the presence of statistically infrequent high crest-factor occurrences in the received signal to be driven onto the loop.
One solution is to power the amplifiers from a single bipolar high voltage rail, typically +15V and -15V. The voltage being chosen so that the signal being driven is not clipped at the high crest factor occurrences. In this cases a large portion of the power is dissipated in the real source resistance used to match the impedance on the line.
Another solution uses a differential voltage rail, which requires the addition of a second voltage rail thereby increasing the required rails from two to four. The rails could be +12V(or +15V), -12V(or -15V), +5V and -5V. The high voltage rails handle the high crest factor occurrences, whereas the low voltage rails handle the normal signal condition. For example Texas Instruments THS6032 is an ultra low-power, differential line driver designed for Central Office (CO) ADSL applications. The THS6032 has Class-G architecture that enables the device to be powered from both low voltage and high voltage power supplies. The voltage rails are switched when low probability peak power events occur. In this way the low voltage rails are used most of the time thereby keeping power low. The high voltage rail is switched to gracefully when large signal levels are transmitted. In this way the high power dissipation is only encountered when actually required. The application of such an amplifier in xDSL modems is disclosed in U.S. Pat. No. 5,898,342.
The above solution is costly both from a direct cost and from a board space point of view. Accordingly there is a need for a low power drive stage that is capable of handling high crest factors while mitigating at least some of the above disadvantages.